1. Field of the Invention
The invention relates generally to the field of mapping spatial distribution with respect to time of fluids injected into subsurface rock formations. More specifically, the invention relates to methods for determining spatial distribution of an injected fluid in rock pore spaces when electrical conductivity contrast between the native (connate) fluid and the injected fluid is insufficient for conductivity based fluid mapping.
2. Background Art
Fluids are injected into subsurface rock formations for a number of different purposes, for example, to flush hydrocarbon from pore spaces in the formations and to move contamination or help remediate rock formations in environmental clean-up situations. For many such purposes, it is desirable that the spatial distribution of the injected fluid is determined or mapped as a function of time. If there is sufficient electrical resistivity (conductivity) contrast between the injected fluid and the existing fluid in the rock formation pore spaces, then the electrical resistivity of the formation containing the injected fluid will differ from that of the surrounding formation containing native fluid. In such cases, surface or borehole electromagnetic (“EM”) measurement methods (e.g., galvanic or induction) can be used to map the spatial distribution of fluid movement and the fluid front with respect to time.
However, in a number of instances there is not a large enough resistivity/conductivity contrast between the injected fluid and fluid in the surrounding rock formation in order to use EM mapping techniques known in the art.
It is also known in the art to impart injected fluid with detectable characteristics to physically distinguish the injected fluid from the fluid already present in the pore spaces of an injection formation. One example of such technique is to include a radioisotope in the injected fluid that can be monitored by a suitable radiation detector. However, use of radioisotopes may be prohibited in certain areas, and their range of detectability through rock formations is generally limited to a distance of about one foot from the radiation detector.
There is a need for other methods to be able to map the spatial distribution of the injected fluid.